The present disclosure relates to optical computing devices that employ integrated computational elements and, more particularly, to improved systems and methods for calibrating integrated computational elements.
Optical computing devices, also commonly referred to as “opticoanalytical devices,” can be used to analyze and monitor a substance in real time. Such optical computing devices will often employ an optical processing element that optically interacts with the substance or a sample thereof to determine quantitative and/or qualitative values of one or more physical or chemical properties of the substance. The optical element may be, for example, an integrated computational element (ICE), also known as a multivariate optical element (MOE), which is essentially an optical interference filter that can be designed to operate over a continuum of wavelengths in the electromagnetic spectrum from the UV to mid-infrared (MIR) ranges, or any sub-set of that region. Electromagnetic radiation that optically interacts with a substance is changed and filtered by the ICE so as to be readable by a detector, such that an output of the detector can be correlated to the physical or chemical property of the substance being analyzed.
An ICE (hereafter “ICE component”) typically includes a plurality of optical layers consisting of various materials whose index of refraction and size (e.g., thickness) may vary between each layer. An ICE design refers to the number and thickness of the respective layers of the ICE component. The layers may be strategically deposited and sized so as to selectively pass predetermined fractions of electromagnetic radiation at different wavelengths configured to substantially mimic a regression vector corresponding to a particular physical or chemical property of interest of a substance. Accordingly, an ICE design will exhibit a transmission function that is weighted with respect to wavelength. As a result, the output light intensity from the ICE component conveyed to the detector may be related to the physical or chemical property of interest for the substance.
After manufacture and before it is put into field use, each ICE component must be carefully calibrated against known calibration fluids for all temperature and pressure ranges expected to be encountered in the field. This calibration process, however, can be quite complicated and time consuming. For instance, the time required to calibrate an ICE component is multiplied by the number of calibration fluids used in the calibration system. In an example system that employs ICE to measure reservoir fluids downhole, there are typically five calibration fluids used, but this number can increase depending on time and required accuracy. The time required to calibrate an ICE component is also multiplied by the number of gas charges (for gas-to-oil ratio) applied to each calibration fluid. For instance, gas-oil-ratio (GOR) calibration requires gas charging stages that entail long set-up times in order to ensure uniform mixing between the calibration fluid in the calibration system and any newly added gas volumes.
Lastly, the time required to calibrate an ICE component is also multiplied by the number of desired temperature and pressure data points used. At each desired temperature and pressure data point, there is a lengthy delay before the temperature and pressure control systems are able to equilibrate the calibration fluid to a steady state before data can be obtained. All of these requirements can result in weeks of continuously running the calibration routine in order to calibrate a single set of ICE components for field operations.